Drosophila melanogaster male somatic cells feminized solely by TraF can collaborate with female germ cells to make functional eggs

Female differentiation of Drosophila germ cells is induced by cell-nonautonomous signals generated in the gonadal soma that work with germ-cell-autononmous signals determined by germ-cell X chromosome dose. Generation of the nonautonomous ferminizing signals was known to involve female-specific protein encoded by the master sex-determination gene Sex-lethal (Sxl) acting on its switch-gene target transformer (tra) to produce Tra(F) protein. However, it was not known whether Sxl's action on tra alone would suffice to trigger a fully ferminizing nonautonomous signal. We developed a constitutively feminizing tra transgene that allowed us to answer this question. In gynanders (XX//XO mosaics) feminized by this Tra(F) transgene, functionally Sxl(-) haplo-X (chromosomally male) somatic cells collaborated successfully with diplo-X (chromosomally female) germ cells to make functional eggs. The fertility of such gynanders shows not only that Tra(F) is sufficient to elicit a fully feminizing nonautonomous signal, but also that haplo-X somatic cells can execute all other somatic functions required lot oogenesis, despite the fact that their genome is not expected to be dosage compensated for such diplo-X-specific functions. The unexpected observation that some Tra(F)-ferninized gynanders failed to lay their eggs showed there to be diplo-X cells outside the gonad for which Tra(F)-feminized haplo-X cells cannot substitute.